题目

       Saunders has since become the youngest person to ski alone to the North Pole, and he’s skied more of the North Pole by himself than any other British man. His old schoolmates would not believe the change.        Next October, Saunders, 27, heads south from the coast of Antarctica to the South Pole and back, a 2900-kilometre journey that has never been completed on skis. 56. What change happened to Saunders after he was 15 years old? A. He became good at most sports.                   B. He began to exercise his body. C. He joined a sports team.  �（10分） 细胞分裂是生物体生长、发育、繁殖和遗传的基础。根据下图回答相关问题（1） 中心法则的内容是                             （用文字和箭头表示）它描述了遗传信息的传递方向，其内容主要体现在细胞分裂的                时期。（2） 图中表示了中心法则中的DNA复制过程，科学家在研究此过程所运用的技术方法是          ，此过程中发生的可遗传变异是            , 原因是                 。（3） 如果图左是卵原细胞，产生基因型为Ab的卵细胞,该卵原细胞在减数分裂中是否有交叉交换的发生?                  ,为什么?                        。（4） 如果这个卵原细胞形成基因型为AB的卵细胞，则其形成的第二极体的基因型为                 。